The present invention relates to a negative-working heat-sensitive material which is suitable for making a lithographic printing plate by direct-to-plate recording and to a method for imaging said heat-mode recording material by means of an infrared laser.
Lithographic printing is the process of printing from specially prepared surfaces, which contain a lithographic image consisting of areas that are capable of accepting ink (oleophilic areas) and areas that do not accept ink but are water-accepting (hydrophilic areas). In so-called wet lithographic printing methods, both water or an aqueous dampening liquid (also called fountain solution) and ink are applied to the plate surface that contains the hydrophilic and oleophilic areas. The hydrophilic areas are soaked with water or the dampening liquid and are thereby rendered oleophobic.
Various heat-mode plate materials are known which can be used as a lithographic master for printing with greasy inks. Ablative plates are the best known examples of so-called processless plates, i.e. plates which do not require any processing and therefore can be used as a printing plate immediately after exposure. The heat, which is generated in the recording layer of such ablative plates by light absorption of a laser beam, removes a hydrophilic or oleophilic topcoat to expose an underlying oleophilic respectively hydrophilic surface, thereby obtaining the necessary differentiation of ink-acceptance between the image (printing) and non-image or background (non-printing) areas.
For example DE-A-2 448 325 discloses a laser heat-mode xe2x80x98direct negativexe2x80x99 printing plate comprising e.g. a polyester film support provided with a hydrophilic surface layer. The disclosed heat-mode recording material is imaged using an Argon laser thereby rendering the exposed areas oleophilic. An offset printing plate is thus obtained which can be used on an printing press without further processing. The plate is called a xe2x80x98direct negativexe2x80x99 plate because it is suitable for direct exposure by a laser beam (xe2x80x9ccomputer-to-platexe2x80x9d, no film mask required) and because the areas of the recording material that have been exposed to the laser are rendered ink-accepting and define the image areas, i.e. the printing areas.
Other disclosures in DE-A-2 448 325 concern xe2x80x9cdirect negativexe2x80x9d printing plates comprising e.g. hydrophilic aluminum support coated with a water soluble laser light (Argon-488 nm) absorbing dye or with a coating based on a mixture of hydrophilic polymer and laser light absorbing dye (Argon-488 nm). Further examples about heat-mode recording materials for preparing xe2x80x9cdirect negativexe2x80x9d printing plates have been described in e.g. DE-A-2 607 207, DD-A-213 530, DD-A-217 645 and DD-A-217 914. These documents disclose heat-mode recording materials that contain an anodized aluminum support and a hydrophilic recording layer provided thereon. Laser exposure renders the exposed areas insoluble and ink-receptive, whereas the non-exposed areas remain hydrophilic and water-soluble. Such plates can also be used directly on the press without processing, because the non-exposed areas are removed by the dampening liquid during printing, thereby revealing the anodized aluminum support.
DD-A-155 407 discloses a processless heat-mode xe2x80x98direct negativexe2x80x99 printing plate where a hydrophilic aluminum oxide layer is rendered oleophilic by direct laser heat-mode imaging.
The above heat-mode xe2x80x98direct negativexe2x80x99 lithographic printing plate are characterized by a low recording speed and/or the obtained plates are of poor quality and durability.
EP-A-580 393 discloses an ablative lithographic printing plate directly imageable by laser discharge, the plate comprising a topmost first layer and a second layer underlying the first layer wherein the first layer is characterized by efficient absorption of infrared radiation and the first and second layer exhibit different affinities for at least one printing liquid.
EP-A-683 728 discloses a heat-mode recording material comprising on a support having an ink receptive surface or being coated with an ink receptive layer a substance capable of converting light into heat and a hardened hydrophilic surface layer having a thickness not more than 3 xcexcm.
U.S. Pat. No. 4,034,183 describes a processless lithographic plate that comprises a light-absorbing hydrophilic top layer coated on a support which is exposed to a laser beam to convert the absorber from an ink repelling to an ink receiving state. All of the examples and teachings require a high power laser, and the run lengths of the resulting lithographic plates are limited.
U.S. Pat. No. 3,832,948 describes both a printing plate with a hydrophilic layer that may be ablated by strong light from a hydrophobic support and also a printing plate with a hydrophobic layer that may be ablated from a hydrophilic support. However, no examples are given,
U.S. Pat. No. 3,964,389 describes a processless printing plate based on the principle of laser transfer of material. This process is very sensitive to transfer defects and requires an additional donor sheet.
U.S. Pat. No. 4,054,094 describes a process for making a lithographic printing plate by using a laser beam to etch away a thin top coating of polysilicic acid on a polyester base, thereby rendering the exposed areas receptive to ink. No details of run length or print quality are given, but it is expected that an non-crosslinked polymer such as polysilicic acid will wear off rapidly and give a short run length.
U.S. Pat. No. 4,081,572 describes a method for preparing a printing master on a substrate by coating the substrate with a hydrophilic polyamic acid and then image-wise converting the polyamic acid to melanophilic polyimide with heat from a flash lamp or a laser. No details of run length, image quality or ink/water balance are given.
Japanese Kokai No. 55/105560 describes a method of preparation of a lithographic printing plate by laser beam removal of a hydrophilic layer coated on a melanophilic support, in which the hydrophilic layer contains colloidal silica, colloidal alumina, a carboxylic acid, or a salt of a carboxylic acid. The only examples given use colloidal alumina alone, or zinc acetate alone, with no crosslinkers or addenda. No details are given for the ink/water balance or limiting run length.
WO 92/09934 describes and broadly claims any photosensitive composition containing a photo acid generator, and a polymer with acid labile tetrahydropyranyl groups. This would include a hydrophobic/hydrophilic switching lithographic plate composition. However, such a polymeric switch is known to give weak differentiation between hydrophilic and oleophilic areas.
All the examples mentioned in the prior art fail to prepare a processless direct imageable printing plate which has a high sensitivity, good start-up behaviour and offers a high run length.
Unpublished EP-A no. 99202109, filed on 29.06.99, discloses a negative-working heat-sensitive material for making lithographic plates comprising in the order given a lithographic base having a hydrophilic surface, an oleophilic imaging layer and a cross-linked hydrophilic upper layer. The heat generated during exposure in the imaging layer removes the hydrophilic upper layer by ablation. However, the water-acceptance of the non-exposed areas is insufficient and, as a result, the plate has an inferior start-up behaviour, i.e. the non-exposed areas to a certain extent accept ink (a defect known as xe2x80x9ctoningxe2x80x9d) while printing the first 10 to 50 copies, which are lost due to bad print quality.
It is an object of the present invention to provide a processless material that is suitable for heat-mode direct-to-plate recording and is characterized by a high lithographic quality, especially with regard to start-up behaviour. This object is realized by the material defined in claim 1. Preferred embodiments thereof are defined in the dependent claims.
The lithographic printing plate of the present invention comprises in the order given a lithographic base having a hydrophilic surface, an oleophilic imaging layer and a cross-linked hydrophilic upper layer.
The cross-linked hydrophilic upper layer is preferably coated from aqueous compositions containing hydrophilic binders having free reactive groups including e.g. hydroxyl, carboxyl, hydroxyethyl, hydroxypropyl, amino, aminoethyl, aminopropyl, carboxymethyl, etc., along with suitable crosslinking or modifying agents including e.g. hydrophilic organotitanium reagents, aluminoformyl acetate, dimethylol urea, melamines, aldehydes, hydrolyzed tetraalkyl orthosilicate, etc. Suitable hydrophilic binders for use in the upper layer may be selected from the group consisting of gum arabic, casein, gelatin, starch derivatives, carboxymethyl cellulose and the salts thereof, cellulose acetate, sodium alginate, vinyl acetate-maleic acid copolymers, styrene-maleic acid copolymers, polyacrylic acids and salts thereof, polymethacrylic acids and salts thereof, hydroxyethylene polymers, polyethylene glycols, hydroxypropylene polymers, polyvinyl alcohols and hydrolyzed polyvinylacetate having a hydrolyzation degree of at least 60% by weight and more preferably at least 80% by weight.
Hydrophilic layers containing polyvinylalcohol or polyvinylacetate hydrolyzed to an extent of at least 60% by weight hardened with a tetraalkyl orthosilicate, e.g. tetraethyl orthosilicate or tetramethyl orthosilicate, as disclosed in e.g. U.S. Pat. No. 3,476,937, are particularly preferred because their use in the present heat-mode recording material results in excellent lithographic printing properties.
A further suitable cross-linked hydrophilic layer is disclosed in EP-A- 514 990. The layer disclosed in this EP-application comprises the hardening reaction product of a (co)polymer containing amine or amide functions having at least one free hydrogen (e.g. amino modified dextrane) and aldehyde.
The cross-linked hydrophilic upper layer preferably also contains substances that increase the mechanical strength and the porosity of the layer e.g. metal oxide colloid particles which are particles of titanium dioxide or other metal oxides. Incorporation of these particles gives the surface of the cross-linked hydrophilic layer a uniform rough texture consisting of microscopic hills and valleys. These particles are preferably oxides or hydroxides of beryllium, magnesium, aluminum, silicon, gadolinium, germanium, arsenic, indium, tin, antimony, tellurium, lead, bismuth or a transition metal. Particularly preferable colloid particles are oxides or hydroxides of aluminum, silicon, zirconium and titanium, used in 20 to 95% by weight of the hydrophilic layer, more preferably in 30 to 90% by weight of the hydrophilic layer.
According to the present invention, the addition to the cross-linked hydrophilic upper layer of an organic compound derived from sulfonic acid (xe2x80x94SO3H), sulfuric acid(xe2x80x94Oxe2x80x94SO3H) phosphoric acid(xe2x80x94Oxe2x80x94PO3H2) or phosphonic acid (xe2x80x94PO3H2) gives rise to an improved start-up behaviour, which is comparable to a conventional plate wherein an electrochemically grained and anodized lithographic aluminum substrate defines the non-printing areas. This effect is obtained even when only a small amount of said organic compound is added to the cross-linked hydrophilic upper layer, e.g. an amount between 0.5% and 25% by weight of the dry hydrophilic layer and more preferably between 1% and 15% by weight.
More particularly, said organic compound comprises an organic radical corresponding to one of the following formula:
R1xe2x80x94(O)nxe2x80x94PO3A2xe2x80x83xe2x80x83(I)
R2xe2x80x94(O)nxe2x80x94SO3Axe2x80x83xe2x80x83(II)
wherein n is 0 or 1 and A is hydrogen, a counter ion or an alkyl group. In formula I, both A groups can have any of the latter meanings independently from one another or can together represent a divalent counter ion or an alkylene group. R1 and R2 are an organic radical. R1 can be a low molecular or a macromolecular radical. R2 is a macromolecular organic radical. The term xe2x80x9cmacromolecular radicalxe2x80x9d comprises polymers, copolymers, dendrimers, hyperbranched polymers, oligomers and multifunctional compounds preferably having a molecular weight higher than 500 g/mol. Preferred examples of such macromolecular compounds are: polystyrene sulfonic acid, polyvinylphosphonic acid, polyvinyl-methylphosphonic acid, phosphoric acid esters of polyvinyl alcohol, polyvinylsulfonic acid, polyvinylbenzenesulfonic acid, sulfuric acid esters of polyvinyl alcohol, acetals of polyvinyl alcohols formed by reaction with a sulfonated aliphatic aldehyde and acetals of poly(vinylalcohols) formed by reaction with a sulfonated aromatic aldehyde.
Other preferred examples are polymers or copolymers comprising the following monomers: p-vinylbenzylphosphonic acid, 2-propenyl-phosphonic acid diethyl ester, [2-methyl-2-[(1-oxo-2-propenyl)-amino]propyl]-phosphonic acid, xcex1-phenylvinylphosphonic acid, vinyl phosphonic acid, phosphonated maleic anhydride, phosphonated acrylates or methacrylates, dimethyl vinylphosphonate, 2-propenyl phosphonic acid, phosphonomethylated acrylamides, phosphono-methylated vinylamines, vinyl aminomethylene phosphonic acid, 1-phenyl vinyl phosphonic acid, vinyl phosphonic acid, (acrylamido methylpropyl) phosphonic acid, Methyl vinylphosphonate, monovinyl ester of phosphoric acid (vinyl phosphate), monoallyl ester of phosphoric acid (allyl phosphate), 2-propenyl-phosphonic acid (allylphosphonic acid), 2-methyl-, 2-[(3-phosphonopropyl)thio]ethyl ester of 2-propenoic acid, 2-[(3-phosphonopropyl)thio]ethyl ester of 2-propenoic acid, 2-Acrylamido-2-methyl-1-propanesulfonic acid (AMPS), sulfo isophtalic acid, sulfo ethyl methacrylate, 2-(sodiosulfo) ethylmethacrylate, (xcfx89-sulfoxyalkyl) acrylates or methacrylates, styrene sulfonic acid, diazosulphonate monomers like methacrylamidophenyl diazosulphonate, allyl ethoxy sulphates, 1-allyloxy-2-hydroxypropyl sulphonate, vinylsulfonic acid, Sulfuric acid monoethenyl ester (vinyl sulfuric acid), mono-2-propenyl ester of sulfuric acid, Sulfuric acid monovinyl ester (vinyl sulfuric acid), Sulfatoethyl methacrylate (Bisomer SEM), Methacrylic acid 2-hydroxyethanesulfonic acid ester (Sulfoethyl methacrylate SEM) and their salts. Typical useful sulfonated polymers can be obtained from e.g. Alco Chemical (division of National Starch and Chemical Company) e.g. with the trade names: Versa TL, Narlex D, Aquatreat AR-540, Aquatreat AR-546, Aquatreat AR-545. Other applicable sulfonic acid polymers are e.g. methylene coupled condensation products of arylsulphonic acid (e.g. available from Bayer under the trade name Baykanol) or sulpho isophtalic acid based polyesters e.g. available from Eastman Chemical Company or Agfa. Other useable sulfate containing polymers are modified polyvinylalcohols, e.g. Poval S2217 (a PVA copolymer with AMPS sodium salt, obtained from Kuraray) and Gohseran L3266 (a PVA copolymer with propene sulphonic acid sodium salt). Other useful polymers are polymer derivatives obtained from polymer analogous reactions such as phosphono-methylations, phosphonylations, phophonations, sulphonations or sulphonylations such as e.g. dextan sulphate (available from Pharmacia Fine Chemicals) or sulphonated alkylene oxide containing polymers. Typical examples of phonomethylated polymers can be derived from polyamines, polyalkylene imines, polyacrylamides, polypropylene imine dendrimers, polyamido amines, oligo(alkylene imines), ect. Besides traditional homopolymers and copolymers also branched macromolecules like hyperbranched polymers or dendrimers could be applied.
In particular multifunctional phosphonates with the trade name Briquest are highly preferred. One of the suitable examples is Briquest 8106/25S=[[3,6,9,12-tetrakis-(phosphonomethyl)-3,6,9,12-tetraaza tetradecane-1,14-diyl]-bis[nitrilobis(methylene)]]tetrakis-Phosphonic acid sodium salt: 
wherein x is an integer between zero and the total number of acid protons in the molecule. Other suitable Briquest grades are: Briquest 3010-25K ([(oxidonitrilo)tris(methylene)]tris-Phosphonic acid, potassium salt), Briquest 281-25S ([[(2-ethylhexyl) imino]bis (methylene) ]bis-Phosphonic acid, sodium salt), Briquest 422-33N ([1,2-ethanediylbis[nitrilobis(methylene)]]-tetrakis-Phosphonic acidtetraammonium salt), Briquest 785 ([[(phosphonomethyl)-imino]bis [2, 1-ethanediyl [(phosphonomethyl)-imino]-2,1-ethanediylnitrilobis (methylene)]] Phosphonic acid, tetrakis-, sodium salt), Briquest ADPA 60AW ((1-hydroxyethylidene)-bis-Phosphonic acid, potassium salt), Briquest 221 ([[bis[2-[bis (phosphonomethyl)-amino]ethyl]amino]methyl]-Phosphonic acid, sodium salt), Briquest 543 ([[(2-hydroxyethyl)imino]dimethylene]di-Phosphonic acid, sodium salt), Briquest 301 Low AM ([nitrilotris (methylene)]tris-Phosphonic acid, sodium salt) Briquest 301-50A ([nitrilotris (methylene)]tris-Phosphonic acid), Briquest ADPA 60A ((1-hydroxyethylidene)bis-)Phosphonic acid). Besides products with trades names Briquest also similar products with trade names Dequest, Masquol, Sequion, Ferriox, Mayoquest, Tecquest, Turpinal, Wayplex, Xidiphone or Xydiphone might be used.
Structural formulas of suitable Briquest types are given below (wherein x has the same meaning as defined previously):
Briquest 3010-25K: 
Briquest 281-25S: 
Briquest 422-33N: 
Briquest 785: 
Briquest ADPA 60AW: 
Briquest 221: 
Briquest 301-50A: 
Briquest 301 Low AM: 
Briquest 543: 
The cross-linked hydrophilic upper layer is preferably coated at a dry thickness of 0.3 to 5 xcexcm, more preferably at a dry thickness of 0.5 to 3 xcexcm.
The cross-linked hydrophilic upper layer may further comprise additional substances such as e.g. plasticizers, pigments, dyes etc. The cross-linked hydrophilic upper layer may also contain an IR-absorbing compound in order to increase the IR-sensitivity. Particular examples of suitable cross-linked hydrophilic layers for use in accordance with the present invention are disclosed in EP-A-601 240, GB-P-1 419 512, FR-P-2 300 354, U.S. Pat. No. 3,971,660, and U.S. Pat. No. 4,284,705.
The oleophilic imaging layer comprises a binder and a compound capable of converting light into heat.
Suitable compounds capable of converting light into heat are preferably infrared absorbing components having an absorption in the wavelength range of the light source used for image-wise exposure. Particularly useful compounds are for example dyes and in particular infrared dyes as disclosed in EP-A-908 307 and pigments and in particular infrared pigments such as carbon black, metal carbides, borides, nitrides, carbonitrides and bronze-structured oxides. It is also possible to use conductive polymer dispersion such as polypyrrole, polyaniline, or polythiophene-based conductive polymer dispersions. Carbon black or graphite yield very good and favorable results.
The binder of the oleophilic imaging layer is preferably selected from the group consisting of polyvinyl chloride, polyesters, polyurethanes, novolac, polyvinyl carbazole, or copolymers or mixtures thereof. In a most preferred embodiment, the binder itself is heat-sensitive: e.g. a self-oxidizing polymer containing nitrate ester groups such as cellulose nitrate as disclosed in GB-P-1 316 398 and DE-A-2 512 038; a polymer containing carbonate groups such as polyalkylene carbonate; or a polymer containing covalently bound chlorine such as polyvinylidene chloride. Also substances containing azo or azide groups, capable of liberating N2 upon heating are favorably used.
The oleophilic imaging layer preferably also contains transition metal complexes of an organic acid. Preferred examples of such transition metal complexes are the chromium complexes of organic acids, such as the products sold under the QUILON trade name by Dupont Corporation, e.g. QUILON C, a 25 to 30% by weight solution of the Werner complex of trivalent chromium and myristic or stearic acid in isopropyl alcohol, as described in Quilon chrome Complexes, Dupont Corporation, April, 1992.
The dry coating weight of the oleophilic imaging layer is preferably between 0.10 and 0.75 g/m2, more preferably between 0.15 and 0.50 g/m2. If the oleophilic imaging layer is too thin ( less than 0.1 g/m2), the oleophilicity of the exposed areas is low (due to the underlying lithographic base) and the run length is mainly limited by the exposed areas. If the IR-sensitive oleophilic layer is too thick ( greater than 0.75 g/m2) the effect of the hydrophilic surface of the lithographic base is lost and the run length may be limited by the non-exposed areas due to toning.
According to the present invention, the lithographic base may be an anodized aluminum support. A particularly preferred lithographic base is an electrochemically grained and anodized aluminum support. The anodized aluminum support may be treated to improve the hydrophilic properties of its surface. For example, the aluminum support may be silicated by treating its surface with a sodium silicate solution at elevated temperature, e.g. 95xc2x0 C. Alternatively, a phosphate treatment may be applied which involves treating the aluminum oxide surface with a phosphate solution that may further contain an inorganic fluoride. Further, the aluminum oxide surface may be rinsed with a citric acid or citrate solution. This treatment may be carried out at room temperature or may be carried out at a slightly elevated temperature of about 30 to 50xc2x0 C. A further interesting treatment involves rinsing the aluminum oxide surface with a bicarbonate solution. Still further, the aluminum oxide surface may be treated with polyvinylphosphonic acid, polyvinylmethylphosphonic acid, phosphoric acid esters of polyvinyl alcohol, polyvinylsulfonic acid, polyvinylbenzenesulfonic acid, sulfuric acid esters of polyvinyl alcohol, and acetals of polyvinyl alcohols formed by reaction with a sulfonated aliphatic aldehyde It is further evident that one or more of these post treatments may be carried out alone or in combination. More detailed descriptions of these treatments are given in GB-A-1 084 070, DE-A-4 423 140, DE-A-4 417 907, EP-A-659 909, EP-A-537 633, DE-A-4 001 466, EP-A-292 801, EP-A-291 760 and U.S. Pat. No. 4,458,005.
According to another mode in connection with the present invention, the lithographic base can also be a flexible support, which is provided with a hydrophilic layer, hereinafter called xe2x80x98base layerxe2x80x99. The flexible support is e.g. paper, plastic film or aluminum. The base layer is preferably a cross-linked hydrophilic layer obtained from a hydrophilic binder cross-linked with a hardening agent such as formaldehyde, glyoxal, polyisocyanate or a hydrolyzed tetra-alkylorthosilicate. The latter is particularly preferred.
The hydrophilic binder for use in the base layer is e.g. a hydrophilic (co)polymer such as homopolymers and copolymers of vinyl alcohol, acrylamide, methylol acrylamide, methylol methacrylamide, acrylate acid, methacrylate acid, hydroxyethyl acrylate, hydroxyethyl methacrylate or maleic anhydride/vinylmethylether copolymers. The hydrophilicity of the (co)polymer or (co)polymer mixture used is preferably the same as or higher than the hydrophilicity of polyvinyl acetate hydrolyzed to at least an extent of 60% by weight, preferably 80% by weight.
The amount of hardening agent, in particular tetraalkyl orthosilicate, is preferably at least 0.2 parts per part by weight of hydrophilic binder, more preferably between 0.5 and 5 parts by weight, most preferably between 1 parts and 3 parts by weight.
The hydrophilic base layer may also contain substances that increase the mechanical strength and the porosity of the layer. For this purpose colloidal silica may be used. The colloidal silica employed may be in the form of any commercially available water dispersion of colloidal silica for example having an average particle size up to 40 nm, e.g. 20 nm. In addition inert particles of larger size than the colloidal silica may be added e.g. silica prepared according to Stober as described in J. Colloid and Interface Sci., Vol. 26, 1968, pages 62 to 69 or alumina particles or particles having an average diameter of at least 100 nm which are particles of titanium dioxide or other heavy metal oxides. By incorporating these particles the surface of the hydrophilic base layer is given a uniform rough texture consisting of microscopic hills and valleys, which serve as storage places for water in background areas.
The thickness of the hydrophilic base layer may vary in the range of 0.2 to 25 xcexcm and is preferably 1 to 10 xcexcm.
Particular examples of suitable hydrophilic base layers for use in accordance with the present invention are disclosed in EP-A-601 240, GB-P-1 419 512, FR-P-2 300 354, U.S. Pat. No. 3,971,660, and U.S. Pat. No. 4,284,705.
As flexible support of a lithographic base in connection with the present embodiment it is particularly preferred to use a plastic film e.g. polyethylene terephthalate film, polyethylene naphthalate film, cellulose acetate film, polystyrene film, polycarbonate film, etc. The plastic film support may be opaque or transparent.
It is particularly preferred to use a film support to which an adhesion improving layer, also called substrate layer, has been provided. Particularly suitable adhesion improving layers for use in accordance with the present invention comprise a hydrophilic binder and colloidal silica as disclosed in EP-A-619 524, EP-A-620 502 and EP-A-619 525. Preferably, the amount of silica in the adhesion improving layer is between 200 mg per m2 and 750 mg per m2. Further, the ratio of silica to hydrophilic binder is preferably more than 1 and the surface area of the colloidal silica is preferably at least 300 m2 per gram, more preferably at least 500 m2 per gram.
Optionally the heat-sensitive imaging material can be covered with an additional hydrophilic layer, provided on top of the hydrophilic upper layer discussed above, which comprises an organic compound containing cationic groups as described in EP-A no. 99202110, filed on Jun. 29, 1999.
In accordance with the method of the present invention, the imaging material is image-wise exposed to cause removal of the cross-linked hydrophilic upper layer and whereby the exposed areas are converted to oleophilic areas while the unexposed areas remain hydrophilic. This is mostly the case when using short pixel dwell times (for example 1 to 100 ns). However when using longer pixel dwell times (for example 1 to 20 xcexcs) the hydrophilic layer may not completely be removed upon exposure. The remaining parts of the hydrophilic layer can then be removed on the press by contact with fountain solution and ink or by an additional wet or dry processing step between the IR-laser exposure and the start-up of the printing process. A suitable dry processing step is e.g. mechanical treatment such as rubbing or brushing the layer with e.g. a cotton path. A preferred additional wet processing step is a gumming step as is commonly used for conventional plates. A gumming step is normally not regarded as a processing step, but rather as a treatment which protects the hydrophilic areas from fingerprints or other contamination which may affect the water-acceptance of these areas. Upon gumming the remaining ablation dust on the plate is removed thereby avoiding contamination of the press. At the same time the hydrophilic areas are covered with a thin layer of the gumming solution inducing a better start-up performance.
Image-wise exposure in connection with the present invention is preferably an image-wise scanning exposure involving the use of a laser or L.E.D. Preferably lasers are used that operate in the infrared or near-infrared, i.e. wavelength range of 700-1500 nm. Most preferred are laser diodes emitting in the near-infrared with an intensity higher than 0.1 mW/xcexcm2.
According to the present invention the plate is then ready for printing without an additional development and can be mounted on the printing press.
According to a further method, the imaging material is first mounted on the printing cylinder of the printing press and then image-wise exposed directly on the press by means of an integrated image recording device. Subsequent to exposure, the imaging material is ready for printing.
The printing plate of the present invention can also be used in the printing process as a seamless sleeve printing plate. In this option the printing plate may be soldered in a cylindrical form by means of a laser. Such cylindrical printing plate which has as diameter the diameter of the print cylinder can be slid on the print cylinder instead of mounting a conventional printing plate. More details on sleeves are given in xe2x80x9cGrafisch Nieuwsxe2x80x9d, 15, 1995, page 4 to 6.
The following example illustrates the present invention without limiting it thereto. All parts and percentages are by weight unless otherwise specified.